This invention relates to the field of ceramic layering onto ceramic substrates for improving the resistance to attack from molten nonferrous metals.
Ceramic coatings have been used for many years to change the surface performance of refractory materials with molten nonferrous metals, such as aluminum and its alloys, magnesium and its alloys, and zinc and its alloys. These coatings often contain boron nitride, which has the characteristic of exhibiting nonwetting/non-stick properties with these molten metals and their drosses/slags. Generally, these coatings contain a high-percentage loading of boron nitride (up to 35% in the liquid coating and 62 to 92% in the dried or sintered coating). The coatings are usually applied by standard house painting techniquesxe2x80x94brushing or air-spraying. Sometimes, dipping or roller-coating is used. The general methods provide a layer of mostly boron nitride, bonded with aluminum oxide, magnesium silicate, magnesium-aluminum silicate, aluminum phosphate, or other compatible binder. The thickness of these coatings is typically 0.003 to 0.010 inch (0.08 to 0.25 mm). Boron nitride powder used for these coatings is typically xe2x88x92325 mesh and often below 10 micrometers particle size with a surface area of 10 to 30 square meters/gram. The ceramic substrates that are painted with these coatings can be nearly fully dense refractories, but are often fibrous structuresxe2x80x94such as vacuum-molded materials that are lightweight. The coatings are very versatile in allowing application to substrates of varying density and composition. The main problem with boron nitride coatings is that they are fairly soft materials. This characteristic results in eventual wear and erosion by molten metal moving across the coated surfaces. Coating removal leads to wetting and sticking of the dross and poor flow of the molten metal in troughs, launders and runners. In molds and other areas, the finish slowly worsens as the boron nitride wears away. This situation is remedied by often re-coating the ceramic components to reapply the boron nitride layer. However, recoating is a tedious, time-consuming, and expensive process.
One possible solution to the erosion problem would be to provide a thicker boron nitride coating. Unfortunately, thick coatings require thorough drying between applications or else moisture is retained in the undercoatings. Such residual moisture often leads to delamination or xe2x80x9cpop-offxe2x80x9d of the coatings when used for the first time. Thick coatings are also more susceptible to xe2x80x9cmud-crackingxe2x80x9d on drying which leads to the coating coming off of the substrate in poorly-attached areas. Another problem with thick boron nitride coatings arises from the thermal expansion differences between the substrate and the coating. Stresses arising from these thermal expansion differences become more pronounced with thicker coatings as they go through temperature changes, leading to spalling of the thick coatings. Thick coatings typically have less strength than thin coatings due to microcracking and low cohesive bonding. Additionally, applying a thick coating is time-consuming for the process worker.
Prior references that mention refractories containing boron nitride include U.S. Pat. No. 5,573,580 to Bartsch et al. and U.S. Pat. Nos. 4,248,752 and 4,174,331 to Myles. The former patent discloses a xe2x80x9crefractory washxe2x80x9d coating stated as a xe2x80x9cmould coating materialxe2x80x9d that may contain boron nitride hollow spheres preferably, or boron nitride fibers. It is generally known in the field that wash/mold coatings are intended for short-term uses, often one casting use. Neither boron nitride spheres nor fiber additions would lead to a boron nitride xe2x80x9cshellxe2x80x9d coated ceramic as our material. The coating of Bartsch is expected to have problems that occur with a simple thick coating, such as xe2x80x9cmud-cracking,xe2x80x9d xe2x80x9cpop-offxe2x80x9d from moisture or microcracking or thermal expansion differences with the substrate. An erosion-resistant nonwetting layer would not occur with the coating of Bartsch, if the boron nitride were used in the form of spheres or fibers, since erosion of soft boron nitride is the reason for the short lifetimes of boron nitride coatings. Additionally, such spheres or fibers of a soft material like boron nitride would not be expected to improve the strength of his coating material. Bartsch also mentions that spheres/fibers melt and crumble when they come in contact with molten metal, thereby limiting the utility of his coating to short term usage.
The moldable material defined by the patents of Myles was designed for adherence to a steel sub-surface. Also, they contain a necessary ingredient of colloidal silica in the range of 7 to 30%, which is known to be highly reactive with molten nonferrous metals, causing wetting and reaction with the metals. Neither Myles or Bartsch were concerned with a xe2x80x9cshellxe2x80x9d layer formed on top of a ceramic structure, said xe2x80x9cshellxe2x80x9d consisting of a matrix of boron nitride which contains a non-boron nitride fiber. Bartsch, in fact, discourages using fibers of any sort due to the difficulty of getting a uniform, smooth layer when fibers are used. Also, there was no effort to achieve the true nonwetting behavior that is desirable with a layer of boron nitride. Myles teaches a very thick composite structure of 1 to 3 inches thickness.
Other patents that discuss various moldable or pumpable compositions that could also contain boron nitride fibers are U.S. Pat. No. 5,053,362 to Chi et al. and U.S. Pat. No. 5,268,031 to Lassiter et al. Chi defines a moldable, clay-like filler that is directed to patching and filling of large holes, typically over 1 inch in thickness, in refractory materialsxe2x80x94to add thermal insulation and thus fix hot spots from furnace liner deterioration. Such a filler is placed into gaps between materials, and thus considerations of delamination, xe2x80x9cpop-off,xe2x80x9d etc. are not a concern with this moldable filler. A necessary ingredient as per Chi is an organic polymer binder, preferably used with chopped fibers and colloidal silica binder. Lassiter et al. describes a pumpable composition, designed for thermal insulation, with the primary distinction of having improved freezing resistance due to the required usage of sodium silicate binder. The pumpable material of Lassiter et al., like the moldable material of Chi et al., is intended for being used to fill xe2x80x9ccavitiesxe2x80x9d in furnace walls etc.xe2x80x94thus again eliminating considerations of delamination, xe2x80x9cpop-off,xe2x80x9d etc. that are required to be addressed with a xe2x80x9cshellxe2x80x9d coating onto a ceramic surface. Also, Lassiter et al. requires a polymer organic component. Thus, the pumpable material of Lassiter would be expected to react readily with molten aluminum, magnesium, and other nonferrous metals due to the presence of sodium silicate binder.
Also, JP 63157747A to Ando et al. utilizes boron nitride as an oxidizing prevention layer on a laminate material with graphite and a ceramic fiber layerxe2x80x94not a matrix of boron nitride which has ceramic fibers added to improve strength and erosion resistance in a boron nitride xe2x80x9cshellxe2x80x9d coated ceramic structure.
It is well known that silicon dioxide, or silica, is reactive with molten aluminum, magnesium, and other nonferrous metals, as noted in U.S. Pat. No. 5,053,362. Whether in powder form or in colloidal form, such silica additions are the basic cause for corrosion and deterioration of refractories that contact molten nonferrous metals. Also, such binders as alkali silicates, including sodium, potassium, or lithium silicates in particular, are known to allow rapid reaction and deterioration in contact with molten nonferrous metalsxe2x80x94as well as allowing wetting of the refractory surface by these nonferrous metals. Molten nonferrous metals will typically reduce silica in the refractory to silicon, which can then dissolve into the molten metal. Concurrently, the nonferrous metal oxidizes (e.g. transforming it to the corresponding oxide such as alumina, magnesia, or a spinel, depending on the metal) and can attach to, and react further with, the refractory. Boron nitride provides a smooth surface that is not reacted with or wetted by molten nonferrous metals. However, it is quite important to minimize the level of any silicon dioxide containing material that is present along with the boron nitride in order to reduce or prevent any reaction and deterioration that is characteristic of the silicates and silica-containing refractories.
None of these references appreciate the need that exists for a boron nitride xe2x80x9cshellxe2x80x9d coated ceramic structure where the xe2x80x9cshellxe2x80x9d consists of a matrix of boron nitride with a strengthening material/filler being ceramic fibers.
The present invention provides a boron nitride paste composition composed of boron nitride powder dispersed in a fully inorganic, non-silicon dioxide binder into which ceramic fibers are added in order to allow a moderately thick boron nitride xe2x80x9cshellxe2x80x9d layer to be bonded to ceramic substrates of different density, composition, and shapes. The layer thickness of the boron nitride xe2x80x9cshellxe2x80x9d material is typically 0.0313 inch to 0.25 inch (0.8 mm to 6.4 mm), which provides a long-lasting, non-wetting/non-stick boron nitride surface for molten nonferrous metals. The preferred composition of the boron nitride paste contains 91% boron nitride paint and 9% ceramic fibers, where the fibers initially are in the dimensional range of up to 4 inches (10 cm) in length and 2 to 3 micrometers (xcexcm) average diameter but are de-clumped such that the average fiber length is greater than 100 micrometers and the clumps are less than {fraction (3/32)} inch (0.24 cm) in diameter. The concentration of the boron nitride in the dried xe2x80x9cshellxe2x80x9d layer is typically in the range of 36 to 78 wt. %, and preferably in the range of 45 to 70 wt. %, providing the nonwetting performance of the moderately thick surface layer. The fibers provide internal reinforcement of the layer, allowing thorough drying without cracking when the layer is built up onto the ceramic, strengthening the layer, improving erosion resistance with molten nonferrous metals, as well as decreasing the thermal expansion mismatch stresses between substrate and the layer.
The standard methods of applying this boron nitride paste to a ceramic substrate are troweling it onto the substrate or dipping the ceramic material into the boron nitride paste. Then, the layer of paste can be evenly smoothed out with an accurately-spaced blade, a process which can be mechanized depending on part geometry. After drying the layer in air at around 100xc2x0 C., it can be burnished/buffedxe2x80x94densifying it somewhat and smoothing it as well as improving its aesthetics. The dried boron nitride xe2x80x9cshellxe2x80x9d coated ceramic can be placed directly into use-environments that will further densify the ceramic and further bond the boron nitride xe2x80x9cshellxe2x80x9d layer onto the substrate ceramic. The dried boron nitride xe2x80x9cshellxe2x80x9d coated ceramic can also be heated up (fired/sintered) prior to placing in service. The ceramic substrate material can be vacuum molded ceramic fiber shapes in an unsintered or sintered state. Much denser ceramic fiber shapes can similarly be used as a substrate. Also, nonfibrous, porous, or dense unsintered or sintered ceramics can be used as the ceramic substrate. Even ceramic substrates not normally considered for contact with molten nonferrous metals can be used due to the superior nonwetting properties of the boron nitride xe2x80x9cshell.xe2x80x9d The combination of substrate and boron nitride xe2x80x9cshellxe2x80x9d significantly increases the performance and durability in molten nonferrous metal contact application when compared to the substrate alone.